250. Many other arrangements were introduced to render the action of the engine regular and uniform, and thus fit it for machinery, and to give it a self-acting power, so that, when once set in motion, the least possible amount of attendance might be necessary to preserve it in continued action. We shall now describe these in detail, commencing with the boiler.

THE BOILER.

251. This is the vessel in which the steam is generated, and forms an exceedingly interesting part of the steam-engine. Nothing can be more beautiful than those adjustments of the boiler and cylinder to each other, by which the engine itself regulates the supply of steam to the cylinder, of water to the boiler, and of heat to the furnace. It thus, in a manner, itself proportions the supply to the demand; and with so much regularity and precision, that the engine in its movements almost rivals the voluntary motions of living beings. The boiler now to be described is that of land engines, acting on the principle of condensation; lowpressure engines or condensing engines, as they are generally termed. The boilers of marine condensing engines are similar in their construction, varying a little in form. The boilers of high-pressure engines will be described afterwards.

252. The boiler is a large vessel formed of sheet-iron plates hammered together. Its shape will be understood from the adjoining figure, representing an end view, and Fig. 15, next page, representing a longitudinal section. The boiler has two principal tubes, one of which conveys to it water to be formed into steam, while the other conveys the steam from the boiler to the cylinder. These are the tubes with the arrows, in Fig. 15. It has gauge-cocks to ascertain the height of the waterin

FIG. 14.

the boiler; a steam-gauge, to indicate the elastic force of the steam; a safety-valve, to give exit to the steam, and prevent the explosion of the boiler; an internal safety-valve, to give access to the air, and prevent the compression of the sides of the boiler by atmospheric pressure, should the elastic force of the steam in the interior be suddenly much diminished from any cause; and a man-hole, by which admission may be had to clean the boiler when necessary. The boiler is placed upon a furnace, supplied with a self

acting damper; and, by Brunton's contrivance, may be made to feed itself with fuel, acccording to the demand.

253. The feed-pipe.-The tube (ii) which conveys the water into the boiler is termed the feed-pipe. It proceeds from a cistern (d) placed above the boiler, and terminates a little lower than half-way between the top and bottom of the boiler. The cistern d is freely supplied with hot water by the pipe rr, which proceeds from the hot well, and conveys (by a pump

worked by the engine) the warm water of the hot well. to the cistern d. The water thus conveyed to the cistern would fall directly down into the boiler by the pipe ii, were it quite open. But, as the demand for steam is not always the same, and it will not therefore do to have a constant quantity of water supplied to the boiler, too much water might enter the boiler; or there might be too little, and the boiler might then be injured by the heat. The feed-pipe, to prevent such irregu larities, and proportion the supply of water to the demand, is rendered self-acting in the following manner. At the bottom of the cistern a valve (e) is placed, which opens upwards when the rod which attaches it to the lever c c is raised, and admits water from the cistern to the tube i i below it. The lever, as will be seen, moves on a fixed point at the upper part of the cistern. To one extremity of the lever a small rod or wire (k) is attached, which passes through an air-tight aperture into the boiler, supporting a stone-float at its extremity. This stone-float is counterpoised by a weight (w) attached to the other end of the lever c c. The weight is such as to balance the float in water, and, accordingly when the level of the water becomes lower from so much being formed into steam, the float will descend (as the weight cannot support it in air.) The float descending will pull down the arm of the lever to which it is attached, elevate the other arm, and thus open the valve in the cistern, so that water will pass from it into the boiler. When the float has been thus raised sufficiently, the weight will then pull down its arm of the lever and shut the valve, so that no more water will enter. In this manner the water is kept always near the same level in the boiler. The cistern is supplied abundantly with water by the pipe (r r) from the hot well, as it is called, the water in which is warm; so that there is a gain of heat proportioned to

the excess of the temperature of the water thus pumped in over the usual temperature of water: this will be explained in the description of the engine.

254. Connected with the feed-pipe of the boiler, there is a contrivance of great ingenuity called the self-acting damper. If the quantity of water supplied be uniform, the amount of steam produced will vary according to the intensity of the fire. If the fire be too strong, more steam will be formed than is required —if weak, too little steam will be produced. By a damper, which contracts or enlarges the throat of the flue of the furnace, the strength of the fire may be increased or diminished, and the quantity of steam will vary accordingly. As the steam in the boiler presses on the water, this water will rise in an open tube to which it has access to a height proportioned to the pressure. The feed-pipe i i is such a tube: in it a weight (f) is suspended, connected by a chain with the damper a. The chain passes through a separate tube in the cistern d, and over two pulleys (b b.) The weight ƒ is such as just to balance the damper a when immersed to a certain extent in water in the tube i i forced up by the elastic force of the steam. Let the weight and damper be adjusted to the required force of the steam, and be in a state of rest. They will remain so until some change in the strength of the steam arises. Should its elastic force be increased, the water will be forced up in the tube; the weight (or a greater part of it) being now supported by water will be lighter in relation to the damper, which is entirely suspended in air; the damper will therefore descend and contract the throat of the flue of the furnace; the draught will thus be diminished, the fire moderated, and less steam formed. Should the elastic force of the steam be diminished, the water will sink in the tube, the weight will descend, the damper will be raised, the